1. Field of the Invention
The present invention relates to a deflection yoke device to be installed to a color CRT provided with an in-line type electron gun.
2. Description of the Related Art
When blue and red lines are displayed on the screen of a color CRT having an in-line type electron gun, misconvergence of a type as shown in FIG. 7 occurs. That is, in an intermediate region I occupying about one half of the effective size of the screen face in the vertical direction, lateral line misconvergence (positive trilemma) occurs such that blue lines 1B and 3B, which are in the first and the third quadrant, deviate obliquely toward the upper side from red lines 1R and 3R, respectively, while blue lines 2B and 4B, in the second and fourth quadrants, deviate obliquely toward the lower side from red lines 2R and 4R, respectively. And, in the upper and the lower regions II, lateral line misconvergence (negative trilemma) occurs such that blue lines 5B and 7B, in the first and the third quadrant, deviate obliquely toward the lower side from red lines 5R and 7R, respectively, while blue lines 6B and 8B, in the second and fourth quadrants, deviate obliquely toward the upper side from red lines 8R and 8R, respectively. In the prior art, it was not possible to solve the positive and negative trilemma problems at the same time. Throughout the specification and claims, the state under which the positive and negative trilemma occur is referred to as inverse trilemma.
As for the means through which the inverse trilemma is reduced, it has been proposed that, as shown in FIG. 8, at least in a part of either one of the horizontal deflection coils 9 and vertical deflection coils 10, that is, at angle regions 11 respectively slanting by about 35.degree. from the horizontal axis x, vacant parts having no winding are provided (Japan Patent Kokoku No. Sho 58-21772). Hereupon, in FIG. 8, "B" indicates an insulating frame, and "C" indicates a core.
By the constitution as described above, for the first quadrant of the screen as shown in FIG. 9, it is possible to generate diagonal direction magnetic field distributions 12H.sub.B, 12H.sub.R, 13H.sub.B and 13H.sub.R that are able to establish a relation: (13.sub.B -13F.sub.R)&gt;&gt;(12F.sub.B -12F.sub.R)(&gt;0), where (12F.sub.B -12F.sub.R) (&gt;0) is the difference between a Lorentz force 12F.sub.B acting on a blue-light-emitting electron beam 12B and a Lorentz force 12F.sub.R acting on a red-light-emitting electron beam 12R in the region I and (13F.sub.B -13F.sub.R), (&gt;0) is the difference between a Lorentz force 13F.sub.B acting on a blue-light-emitting electron beam 13B and a Lorentz force 13F.sub.R, acting on a red-light-emitting electron beam 13R in the regions II.
In such a manner, as shown in FIG. 10, it becomes possible that while making the amount of the negative trilemma T.sub.11 (&lt;0) in the regions II as small as possible and thus reducing a difference (T.sub.1 -T.sub.11) with respect to the amount of the positive trilemma, T.sub.1 (&gt;0), in the region I, by increasing or decreasing the amount of protrusion of the vertical deflection magnetic field toward the electron gun side, the positive and negative trilemma can be reduced to their minima.
The reduction means for reducing the inverse trilemma as stated above, however, only adjusts the magnetic field distribution compromisingly. The reduction means is not able to remove the inverse trilemma completely.
By giving a vertical deflection magnetic field, having a strong barrel distortion to the optimum magnetic field distribution which does not produce pincushion type vertical isotropic astigmatic aberrations 14B, 14R, as shown in FIG. 11, nor barrel type vertical isotropic astigmatic aberrations 15B, 15R, as shown in FIG. 12, on the vertical axis y, a Lorentz force 16F.sub.R in the +y direction acting on a red light emitting electron beam 16R can be made stronger relative to a Lorentz force 16F.sub.B in the +y direction acting on a blue-light-emitting electron beam 16B. And, a Lorentz force 17F.sub.B in the +y direction acting on a blue light emitting electron beam 17B can be made stronger relative to a Lorentz force 17F.sub.R in the +y direction acting on a red light emitting electron beam 17R. Consequently, the positive trilemma occurring in the upper half part in the screen face intermediate region I can be removed.
Next, considering the period while the electron beam is scanning the upper region II of the screen face, if vertical deflection magnetic field distribution has a weaker barrel distortion in comparison with the optimum distribution, it is equivalent relative to the one in which a pincushion distortion is strengthened, and hence it can be explained using a model of a pincushion magnetic field shown in FIG. 14. As is obvious from this figure, a Lorentz force 18F.sub.B in the +y direction acting on a blue-light-emitting electron beam 18B becomes relatively stronger than a Lorentz force 18F.sub.R in the +y direction acting on a red-light-emitting electron beam 18R, whereas a Lorentz force 19F.sub.R in the +y direction acting on a red-light-emitting electron beam 19R becomes relatively stronger than a Lorentz force 19FB in the +y direction acting on a blue-light-emitting electron beam 19B. This fact means that the negative trilemma 5B, 5R, 6B, and 6R, mentioned above, occurring in the upper region II of the screen face can be corrected.
When the electron beam is scanning the lower half part of the screen face also, because of the same reason as that described above, positive trilemma 3B, 3R, 4B, 4R and negative trilemma 7B, 7R, 8B, 8R can be corrected.
In case the electron beam scans the region I and the region II of the screen face, however, on the vertical deflection magnetic field, when the degree of its barrel distortion changes stronger and weaker respectively, as has been described from the above-mentioned optimum distribution, vertical isotropic astigmatic aberrations 20B, 20R, 21B, and 21R of shapes as shown in FIG. 15 occur on the vertical axis y. However, since those vertical isotropic astigmatic aberrations 20B and 20R occurring in the screen face region I are minute, they do not problematically affect the convergence quality. On the other hand, as for those pincushion type vertical isotropic astigmatic aberrations 21B and 21R occurring in the regions II, it is necessary to correct them in a particular way, and such correction can be achieved in a means described below. That is, while the electron beam is scanning the screen face regions II, by generating a quadrupole magnetic field 22, 23, 24, and 25, as shown in FIG. 16, which is synchronized to the vertical deflection period in the vicinity of the opening part on the electron gun side of a deflection yoke, the magnetic field 22 gives a Lorentz force 26F.sub.B to an electron beam 26B in the -x direction, and the magnetic field 23 gives a Lorentz force 27F.sub.R to an electron beam 27R in the +x direction, and thereby a pincushion type vertical isotropic astigmatic aberrations 21B and 21R in the screen face regions II of FIG. 15 can be corrected.